Acidic acrylic copolymers comprising at least one side chain of poly(alkylene glycol) type are occasionally referred to as polymers of comb structure. These are copolymers with a backbone of essentially carboxylic nature onto which are grafted side chains (also known as “pendant chains”) of poly(alkylene glycol) type.
There are at the present time two main routes for preparing these copolymers of comb structure.
The first route for synthesizing these copolymers is copolymerization in the presence of acidic acrylic monomers and polymerizable macromonomers comprising poly(alkylene glycol) chains.
The second route for preparing copolymers of comb structure is the esterification and/or amidification of acidic acrylic polymers with poly(alkylene glycols). According to this route, an acidic acrylic polymer is prepared via conventional polymerization techniques, for example radical polymerization, followed by esterification and/or amidification of this acidic acrylic polymer in the presence of poly(alkylene glycol) compounds. The esterification and amidification processes described in the prior art (for example WO 2007/132 322, WO 2009/090 471, WO 2013/021 029, US 2008/0 119 602) are of discontinuous type with long reaction times.
US 2013/0 274 368 describes a process for esterification of a poly(carboxylic acid) polymer using polyether compounds, said process being of continuous type in an organic solvent and exposing the reagents to microwaves so as to initiate and accelerate the reaction.
The process of the present invention is based on the use of a continuous reactor known as a tubular reactor, for example an extruder. In a tubular reactor, the material may be subjected for a very short time to high temperatures (for example from 100 to 300° C.), with possibilities also of applying high local pressures (50 to 150 bars). The tubular reactor may moreover be equipped with devices for ensuring homogeneity of the molten material during its flow. Extruders are generally used in continuous processes for transforming, for example forming, thermoplastic polymers or thermosetting polymers. During these extrusion and injection processes, the polymers are completely melted so as to give them the desired shape and appearance to obtain the article to be manufactured. Driven by the screws, the material is subjected for a very short time to high temperatures (for example from 100 to 300° C.), to high local pressures (50 to 150 bars) and to a very intense shear. Extrusion is especially suitable for strong materials with a high melting point.
Tubular reactors, such as extruders, may also be used for chemically modifying the molten material. The use of an extruder for performing a chemical reaction is known as reactive extrusion (REX). Specifically as a result of its intrinsic characteristics (temperature, pressure, shear), extrusion offers a suitable solution for reacting two polymer materials, which are otherwise inert or harldy reactive.
Reactive extrusion is moreover described in the context of aqueous radical polymerization. Reference may be made especially to WO 99/58576, which describes solution polymerization for obtaining an acrylic/acrylamide copolymer in the form of a highly crosslinked gel having the consistency of rubber, containing at the core of its three-dimensional network approximately 50% to 70% by weight of water.
Thus, the use of reactive extrusion is described for preparing polymers from raw materials that are relatively insensitive to thermal degradation.
Polymers of poly(alkylene glycol) type, for example poly(ethylene glycol) or PEG, are heat-sensitive. Degradation of these polymers, under the effect of heat, by oxidative decomposition, leads to byproducts such as water, CO2, aldehyde, simple alcohols, acids, glycol esters and vinyl byproducts.
This is likewise the case for polymers of comb type made of pendant chains of poly(alkylene glycol) type. The high temperatures may lead to side reactions, for example release of the pendant chains of the comb polymer and unexpected crosslinking, and thus degrade the quality of the polymer obtained. Substantial crosslinking may also lead to gelation of the whole, to polymers that are finally water-insoluble.